1. Field of the Invention
This invention relates to a mold for vacuum forming plastic sheets and to a method of making such a mold.
2. Prior Art
The vacuum forming of heat-softened sheets of plastic material is well known. Molds suitable for such vacuum forming are typically porous or have holes in a forming surface. A vacuum applied behind the forming surface evacuates air from between the surface and a heat-softened plastic sheet to bring the sheet into conformance with the surface. Tubes for the circulation of cooling fluid may be used in conjunction with the mold to cool the forming surface and the formed plastic sheet. The holes and cooling tubes must be located to avoid interference. For example, where the tubes are cast into the back of a mold, care must be used in drilling the evacuation holes to avoid puncturing a tube.
Typically, the forming surface serves as the top or front of a vacuum chamber that can be evacuated as desired to form and cool heat-softened sheets of plastic. The forming surface must either be quite thick to provide inherent strength or must be reinforced across the back to resist the pressure differential on opposite sides created by the vacuum within the chamber. By way of example, it is known to make porous vacuum molds of refractory material cast into shape and having a substantial thickness to assure rigidity. This construction has the disadvantage of becoming plugged both at the forming surface and within the body of refractory material from dirt, plasticizer and the like. It also makes temperature control at the surface of the mold difficult because the material is usually a poor heat conductor and has substantial mass.
It is also known to use so-called shell molds that are thin relative to the forming area. Such molds can be made, for example, from cast metal, typically aluminum, with integral reinforcing ribs and cooling tubes. Holes are then drilled through the mold and the mold is backed by a vacuum chamber. As another example, such molds can be made by electroforming a sheet-like mold over a pattern of the desired shape. A pattern of holes can then be drilled through the shell or can be formed at the time of electroforming by the process disclosed in the copending application, Ser. No. 241,917, filed Apr. 7, 1972, referred to above. Such shell molds have the inherent disadvantages of being structurally weak because of their thinness and require heavy reinforcement behind the mold to withstand the large and repetitive pressures exerted against the forming surface of the mold when a vacuum is created behind the mold. Cooling tubes are utilized on the back surface of the mold or in the mold body for temperature control, but as already indicated, these must be located to avoid interference with the vacuum holes. This can result in poor temperature control across the mold surface and in many instances the tubes become punctured during the drilling of holes through the mold.
It will be appreciated that temperature control across the surface of a mold is important not only to establish advantageous production cycles, but also to assure the desired physical characteristics of the formed plastic sheet, including the quality of any embossed surface formed on the sheet by the mold surface. Because a sheet of hot plastic, generally between 300.degree. and 500.degree. F., is impressed against the surface of a mold every few seconds in use, the cooling arrangement must not only remove heat from the plastic but also must prevent buildup of residual heat in the mass of the mold and supporting structure. Thus, uniform surface temperature is typically very marginal in molds of large mass or where the cooling flow is not advantageously located across the back of the forming surface.